The present invention is related to the field of high-power fiber lasers and, more particularly, comprises a structure for providing passive phase synchronization and mode discrimination favoring the in-phase supermode for rare-earth-doped fiber lasers and amplifiers.
Due to their intrinsically single transverse-mode wave-guide structure, fiber lasers made from hair-thin high-quality silicate glass are unique in maintaining very high mode quality, even during high-power operation. Moreover, due to their large surface area-to-volume ratio, fiber lasers also possess excellent thermal management properties. Hence, relative to other types of lasers, such devices produce extremely high output beam brightness, defined as the beam power divided by the product of the mode area and the divergence angle of the beam. However, the maximum output power of a single fiber laser is limited by nonlinear optical effects, such as stimulated Brillioun scattering (“SBS”) and stimulated Raman scattering (“SRS”), which occur at threshold power levels that decrease as the light intensity increases.
One solution to avoid the foregoing limitation is to increase the modal field cross-sectional area, which permits increasing the power without increasing the peak intensity to either the SBS or SRS threshold value. Another solution is to combine a large number of emitting fibers into an array structure, which is then mutually phased by any one of a number of techniques so that all of the lasers emit in synchronization, to maximize the on-axis light intensity at a great distance. While it is advantageous for high beam quality to place the emitting fibers as close together as possible, heat generation dictates a minimum separation distance for a given number of emitting fibers and operational power level.
The approaches to accomplish the phasing of an array of fibers are classified into active and passive techniques. In the former, fibers are separated by a distance that is much larger than the fiber diameter in order to allow space for auxiliary instrumentation needed for phase locking. Such a configuration is illustrated in FIG. 1, which is a cross-section taken along the output plane of optic fiber array 10 of the prior art. Array 10 is comprised of independent fibers 12, with each fiber including core 14, inner cladding 16, and outer cladding 18. The phasing is performed by taking phase measurements in the output field and adjusting the phase of each emitting fiber 12 by means of a closed electronic feedback loop.
This design minimizes thermal problems, but yields poor beam quality, although the beam quality may be improved with the aid of an array of micro-lenses. The inherent complexity of this approach, due to the necessary free-space optical components, feedback loops, and the micro-lens array, makes for a relatively fragile, bulky structure of potentially low reliability, and one that is expensive to fabricate. Furthermore, since the number of fibers that have been actively combined to date is certainly less than ten, and probably less than five, despite several years of effort and significant expense, reducing this technique to practice appears problematic.
On the other hand, passive phasing of an array occurs automatically and, since feedback loops are not required, its structure can be much more compact, simpler and robust, and its operation much simplified. For example, the in-phase mode in an array may be favored by using spatial filtering in an external cavity, or by cross-coupling using Talbot or self-Fourier transformation optics.
Another passive design of the prior art is an evanescently coupled multicore array, such as optic fiber 20 shown in FIG. 2. Optic fiber 20 is comprised of core array 22, octagonal inner cladding 24, and annular outer cladding 26. The multicore design exemplified by optic fiber 20 is innately robust and compact, and has worked with as many as nineteen cores in core array 22.
These passive techniques allow much closer packing of the cores, resulting in improved beam quality without requiring additional optics. Furthermore, there is no need for the expensive and complex feedback loop required for active phasing. Nevertheless, there are inherent drawbacks associated with the passive phasing techniques of the prior art.
Firstly, the physical mechanism of the phasing is not well understood, creating uncertainty in predictions for large arrays. A nonlinear coupled-mode model used to explain the experimental results obtained from conventional multicore fibers indicated that the presence of a resonant nonlinear index played a decisive role; however, further development of the theory along those lines has shown that competition by anti-phased modes may become more problematic in larger arrays, so that continued success with larger arrays is not assured.
In addition, evanescent field coupling, as used in optic fiber 20, requires relatively close packing of the cores in core array 22. Furthermore, the coupling is affected by the cores' refractive index, the uniformity of the lattice comprising array 22, and the uniformity of the circular shapes and diameters of the cores. While enhancing beam quality, close packing of the cores in core array 22 severely limits the capacity for thermal management.
There is a need in the art to achieve optical gain in rare-earth-doped fiber lasers having the cost, weight, size, and reliability advantages inherent to passive phasing, while operating at high power, maintaining peak intensity below the threshold for the onset of damage or parasitics generated by nonlinear optical interactions, and limiting the temperature within the core regions to a level that does not damage the device's performance. The present invention fulfills this need in the art.